General purpose polyester resins can be used for most types of molding and laminating. By polyester is meant the polycondensation product of dicarboxylic acids with dihydroxy alcohols in contradistinction to materials known as alkyds. These polyester compounds may be modified by mono-carboxylic acids, monohydroxy alcohols and small amounts of polycarboxylic acids or polyhydroxy alcohols. Such compounds have a wide range of properties and a particular resin may have properties that may make it suitable for one purpose but unsuitable for another. For example, high viscosity resins are useful in vertical layup, where low visocity resins, however, would be required when rapid penetration was desirable.
Because of the wide range of properties that are common to polyester resins their use varies greatly. Polyester resins are used as plasticizers in coatings and plastics to provide toughness and flexability in such materials as cellulose acetate, polystyrene, polyvinyl chloride, etc. Other saturated polyesters, prepared principally from ethylene glycol and terephalic acid, are used as fibers and films. As fibers they are used to produce fabrics. As films they are used because of their strength, toughness and electrical properties. Polyesters are also used to produce polyurethane foams, coating and adhesives.
While the uses of general purpose polyester resins continue to steadily grow, such resins suffer from the drawback that due to their high hydrogen and carbon content they continue to burn fairly readily once ignited. Since much concern has been generated by consumers to reduce the flammability of products such as flammable fabrics, manufacturers have had to find ways to reduce the flammability of polyesters and polyester resins.
The flammability of polyester compounds has been attempted to be reduced in a number of ways. These include the addition of inorganic fillers and organic fire-retardants, chemical modifications of the saturated or unsaturated acid in the resin, chemical modification of the monomer and chemical combination of organo-metallic compounds with the resin.
Inorganic fillers do not necessarily reduce the burning rate of polyester resins. For example, glassfibers often increase the burning rate. Antimony oxides are often used as inorganic fillers to reduce the burning rate, but they impart opacity and must often be used with a chlorinated parafin to increase its effectiveness which is a disadvantage because the often plasticize and thereby upset some of the properties of the resin. They also tend to "sweat-out" of laminates after continious outside exposure resulting in a breakdown of the laminate.
Chemical modification of the unsaturated polyester is often accomplished by the use of tri-.beta. -chloroethyl phosphate. The use, however, of this plasticizing additive quite often lowers the outside durability of the resin. Tetrabromophthalic anhydride can be used rather than tetrachlorophthalic anhydride but its curing characteristics are too unreliable to use them commercially.
Thus, while the techniques used to reduce the flammability of polyester resins do so to some extent, all suffer from some drawback.